CST8177 – Linux IIMore Scripting Techniques

Scripting ProcessExample Script

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arguments to scripts positional parameters input using read exit status test program, also known as [ if statements error messages while loops for loops

scripting so far

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why write scripts in the first place? lists set and shift integer arithmetic scripting process in general

scripting to come

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1.use when an alias gets too complex, impossible◦ aliases cannot use arguments in their

replacement text

2.make new, specialized commands◦ look at /bin/gunzip and /bin/zcat

3.automate long and/or complex tasks◦ example, actually part of the boot system:

/etc/rc.d/rc

◦ backup scripts, bulk user creation, other sysadmin tasks

◦ as we'll see, scripts are the contents of the "regularly scheduled tasks" directories:

/etc/cron.{hourly,daily,monthly}/

Why do we write scripts?

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In the general form of the control statements(examples: if, for, while) in the bash man page we will see the notion of a list◦for name [ in word... ] ; do list ; done◦ if list1; then list2; fi

From the man page for bash:◦ A list is a sequence of one or more pipelines

separated by one of the operators ;, &, &&, or ||, and optionally terminated by one of ;, &, or <newline>.

Lists

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Hey, we already know what most of those mean:◦ ls –al | more # there's that | operator

| means redirect output of one command into another

◦ ls –al > foo & # there's that & operator & means run the command in the background proceed immediately with any command after the &

◦ ls –al ; head myfile # there's the ; operator ; means run one command after another wait for the first command to finish in the foreground, then

when it's finished, proceed with any command after the ;

Lists (cont'd)

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The new ones are ◦ command1 && command2◦ command1 || command2

&& and ||

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&& means AND This means stop at the first command that

has non-zero exit status command1 && command2 is the same as

◦ if command1; then command2; fi◦ example, suppose we want to run process.sh on a

file, and then remove it:process.sh /root/testfile && rm –f /root/testfile if any thing goes wrong with process.sh (non-zero

exit status), the file is NOT removed if the process.sh /root/testfile command goes well

(has zero exit status), THEN the file will be removed

&& example

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|| means OR This means stop at the first command that

returns a zero exit status command1 || command2 is the same as

◦ if command1; then : ; else command2 fi : is a command that does nothing!

◦ example, suppose we are writing a script that needs to process a file, and if that doesn't go well, then the script should terminate abnormally: process /root/testfile || exit 1 if anything goes wrong with process, the script exits

|| example

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Suppose you might qualify for a scholarship: Those who qualify are:

◦ born on the moon, and something if we weren't born on the moon, we don't need to

know what the <something> is; we already know we don't qualify

◦ Algonquin student and something in this case, because we are an Algonquin student,

we do need to know what something is to know whether we qualify

or in other words◦ born on the moon && something◦ Algonquin student && something

&& means "and"

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As soon as we encounter "true", we can stop

You qualify for a $1000 rebate under the following conditions:◦ born on the moon, or ??◦ Algonquin student, or ??

In the first case, we need to know what the exit status of the ?? is, we need to run the ?? command

In the second case, we can stop before running the ?? command

|| means "or", opposite of &&

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&& and || are used with commands that tend to get things done◦ to graduate, you

complete first year && complete second year◦ complete first year is a "command" that gets

things done: you learn the first-year material◦ if you fail first year, you don't attempt second

year -a and –o are used in test, and don't do

things, just affect the exit status of test◦ you are a rich Canadian if

you are Canadian –a you are rich◦ checking whether or not you're Canadian doesn't

get things done – but it does establish a truth value

&& and || versus –a and -o

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In our script we can manipulate the positional parameters

set command to set them shift command to process them in turn

Manipulating positional parameters

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We can set the positional parameters with ◦ set – oneword secondword -e etc

This will set ◦ $1 to oneword◦ $2 to secondword◦ $3 to -e◦ $4 to etc

Set does take options (see bash manpage) and that first – tells set there are no more options (-e in this case is not an option)

set

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shift moves all the arguments to the left shift n moves all the arguments to the left

by n shift

◦ $# is decreased by 1◦ the pre-shift $1 is lost◦ $1 becomes what was in $2◦ $2 becomes what was in $3◦ $3 becomes what was in $4

etc

shift

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The expr command is a common and traditional way to evaluate arithmetic expressions

The arguments to the expr command constitute the arithmetic expression

Doing integer arithmetic

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examples of using expr command:a=`expr 3 + 4` # a gets set to 7a=`expr 3 – 4` # a gets set to -1a=`expr 3 * 4` # a gets set to 12a=`expr 13 / 5` # integer division: 2a=`expr 13 % 5` # remainder: 3 increment the integer in variable aa=`expr $a + 1` # a gets set to a+1

expr examples

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let is not the preferred way to do arithmetic in this course

let arg [arg...]◦ each arg is an arithmetic expression to evaluate◦ see ARITHMETIC EVALUATION of bash man page◦ shell variables are allowed with or without $◦ arithmetic expressions can involve assignment of values

to variables, where "=" is the basic assignment operator examples:let a=1echo “$a”let a=a+1echo “$a”

let

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let seems great, why not use it?◦ because it's not as portable as the alternatives◦ "portable" means you can run your script

unmodified on other systems, maybe running /bin/dash instead of /bin/bash, and it will work just as well

expr is "standard" and works everywhere $((arith)) is what you would use if you

don't want to use expr (and it's faster than expr)◦ x=0◦ x=$(( x + 1 ))◦ x=$(( $x + 1 ))

let not preferred

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What's the difference between ((expression)) and $((expression))

$((expression)) is an expansion like a variable expansion, where $((expression)) is replaced by the results of the evaluated expression

((expression)) is a compound command equivalent to let expression

((expression)) and $((expression))

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You can put $((expression)) where you can put variable expansion like $myvar:

a=0while [ "$a" –lt 10 ]; do echo username: "user$((a+1))" a=$((a+1))done

expression examples

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You can use ((expression)) wherever you would use a command:

((a=0)) # set a to 0((a=a+1)) # increment a to 1a=((a+1)) # syntax errora="((a+1))" # set a to literal string "((a+1))"a="ls" # set a to literal string "ls" In these last two, there is what looks like a

command, ls and ((a+1)), but no command is run

expression examples (cont'd)

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Analyze and understand the problem Write pseudocode to document and

understand the problem Start with a small number of lines in the

script, and get that working properly Add a small amount to the script, and get it

working properly again repeat previous step until script is complete run final tests

Scripting Process

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Example from Sobell, A Practical Guide to Linux, Commands, Editors, and Shell Programming, 2nd Ed, Chapter 10

lnks script:◦ Given a filename, find hard links to that file.◦ Search for hard links under a certain directory if

one is given; otherwise, search for hard links in the current working directory and its subdirectories.

Analyze, understand, pseudocode

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Generally, for any problem, we need to deal with the following three phases

1. Input2. Processing3. Output

lnks script analysis

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Input is the information/data that a script needs before it can do its job

What is the information?◦ name of a file◦ name of a directory

How does our script get its information?◦ the description says "given a filename", and

"directory if one is given"◦ this would imply that those items would be

command line arguments◦ if the description said "ask for", then the script

would prompt for the information after its running

Input Phase

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What's the data that will be processed? What work will our script do?

◦ the script will look for hard links in the file system◦ therefore, the data is the contents of the file

system◦ our script will look for hard links in the file system◦ our script will "read in" this data by running

commands that access the file system (ls, find, etc)

Input Phase (cont'd)

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OK, we now understand the problem◦ take a file name and an optional directory location

as arguments on the command line, and use Linux commands to find hard links at that the specified directory (or the current directory if none was specified) in the file system

Simple, let's just tell the computer to do that!

Input Phase (cont'd)

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Our scripts are like little robots that will do exactly what we tell them to do.

Imagine you move into a new neighborhood and you tell your domestic robot to go to the corner store and bring back a turkey sandwich and a coke

The robot comes back with a pound of butter and a can of crab juice

You aren't happy, but whose fault is it?◦ the store had ham, but no turkey, and pepsi, but

no coke◦ again, whose fault is it that you aren't happy?

The game of "what if?"

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If they don't have coke, then get pepsi, and if they don't have pepsi, then nothing, etc, etc

When we write our scripts, we end up happiest when we specify absolutely every single detail of every possible circumstance

We need to make a conscious effort to predict all possible ways things can go wrong

It's a common script beginner's mistake to think only about everything going right

The major work is dealing properly with what goes wrong

What if?

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What if no directory name is given on command line?◦ use the current directory (hey, this is easy!)

What if no file name is given on the command line?◦ print an error message (but then what?)

What if the given file name is a directory and not a regular file?◦ print an error message and exit

What if too many names are given? What if no hard links are found? What if there are no hard links to the file? What if the directory is unreadable? Doesn't

exist? The more "what ifs" our script can handle, the

better...

What ifs for lnks script

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Speaking of "what if"s... A Test Plan is a document that describes a

process for verifying that the script does the right thing every time no matter what

In simple cases it can be a list of tests, which would have this form:◦ test name◦ short description of what's being tested◦ command (and/or instructions) for running the

test◦ expected result and/or output from running the

test◦ the actual result (filled in when a test plan is

executed)

Test Plans

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As we have seen, the process of understanding the problem and generating a test plan both involve "what if" analysis, and therefore can proceed at the same time

The plan for testing the final result begins to form early (maybe even before the problem is fully understood!)

Test Plan's (cont'd)

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Start small and get that working, and then add to it Let's start with just the "input" phase:

#!/bin/sh –uPATH=/bin:/usr/bin ; export PATHumask 022

# identify links to a file# Usage: $0 file [directory]if [ $# -eq 0 –o $# -gt 2 ]; then echo 1>&2 "$0: Expecting one or two arguments, found $# ($*)" echo 1>&2 "Usage $0 file [directory]" exit 1fi Continue on real Linux system....

Back to lnks script

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After finishing the Input stage, we have our file name and directory under which to search

How do we find the hard links? Think about hard links...

◦ If we know the inode of the file, then all hard links to the same file will have the same inode

◦ We can get the inode of the file using ls◦ We can look for files with the same inode using find

Processing

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Getting the inode number into a variable named inode so we can refer to it as $inode

Consider the output of ls –i Assuming the filename is in $file, Each of

the following will do the trick remember backticks, `command` is

command substitution, same as $(command)◦ inode=`ls -i "$file" | awk '{print $1}'`OR◦ set - `ls –i "$file"`◦ inode=$1OR◦ inode=$(ls –i "$file" | cut –d' ' –f1)

Processing (cont'd)

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Now that we have the inode, how do we find other files with that inode?

This literally has the find command written all over it! (man find)

find "$directory" –inum "$inode" -print

Processing (cont'd)

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How does our script deliver its results? Should the names be put in a special file

somewhere? It depends. In keeping with the Unix filter/pipeline

philosophy, lets have our script print the names on the standard output

Hey, find is already printing the names on its standard output

Recall that the standard output of the script is the same as the standard output of the commands inside the script

Output

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Notice that errors and other messages should be printed on standard error

The "goods" (the actual filenames we were looking for) are printed on standard output

That's how we like it, because now we can redirect standard output to a file, or run it through grep or any other utility, and we won't have those messages mixed in

All of those messages will go to standard error, which we can also redirect elsewhere if we want

Output (cont'd)

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Indenting: look at how the if statements are indented:

if list; then ls –l # indented 4 spacesfi The same would be true of other control

statements:while list; do #indent 4 spacesdone

Indenting

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Variables in our scripts have lower case names

Environment variables are indicated by their UPPER CASE names: SHELL, VISUAL, etc

It's usually best to put variable expansions inside double quotes, to protect any special characters that might be inside the variable:

echo "$somevar"◦ if somevar contained the * character, the double

quotes stop the shell from globbing it

Variables (review)

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The following all mean different things:◦ run the myvar command with two arguments, =

and value:myvar = value

◦ set the myvar variable to have value "", then run the value command with that variable setting in effect

myvar= value◦ run the myvar command with one argument,

namely =value:

myvar =value◦ set the variable myvar to have value value

myvar=value

Variables (cont'd)

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-v option for bash/sh◦ sh –v myscript◦ shell will print each line as its read◦ loop statements are printed once

-x option for bash/sh◦ sh –x myscript◦ shell will display $PS4 prompt and the expanded

command before executing it◦ each loop iteration is shown individually

Debugging shell scripts

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test plan creation involves identifying the "Boundary Conditions"

The "typical case" or "normal case" is a necessary test, and all such cases are considered equivalent (test one and you've tested them all)

Boundary cases are all interesting:◦ present or missing◦ too small, just big enough, typical, almost too big,

too big◦ MINIMUM, ... -1,0,1,... MAXIMUM

testing Boundary Conditions

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Suppose you're looking for 8.3 filenames, where the "main part" is up to 8 characters, and the extension is exactly 3 characters◦ main part of filename

boundaries 0, 1, 8, 9 characters

typical case (only one needed) 2,3,4,5,6,7

◦ extension boundaries

0,3,4 typical case (same as the boundary) 3

Boundary Conditions example

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